This invention relates to a planetary speed reduction gear type starter, in which an internal gear frame and a front bracket are mounted on a front end portion of a yoke for an electric motor, and more particularly to coupling means for the yoke and the front bracket.
FIG. 4 is a sectional view showing essential components of a conventional planetary speed reduction gear type starter disclosed in Japanese Utility Model Unexamined Publication No. Sho. 58-63361. In FIG. 4, a DC motor 1 comprises: a yoke 2 on which a field unit 3 including permanent magnets are mounted; a partition and bearing support 4 secured in the front end portion of the yoke 2; a rotary shaft 5 supported through a bearing 6 by the partition and bearing support 4; and an armature core 7 fixedly mounted on the rotary shaft 5, on which an armature winding assembly 8 is mounted.
Further in FIG. 4, a planetary speed reduction gear unit 10 is designed as follows: An internal gear frame 11 having an internal gear 11a is secured to the inner wall of the yoke 2. A plurality of planet gears are engaged with a sun gear 9 formed on the rotary shaft 5 and with the internal gear 11a. The planet gears revolves around the sun gear while rotating, so as to reduce the speed of rotation. The planet gears 12 are mounted through bearings 15 on support pins 14, which are fixedly embedded in a flange 13a formed at the rear end of an output shaft 13.
The output shaft 13 has a hole formed in its rear end portion. The output shaft 13 is supported by the rotary shaft 5 through a bearing 18 provided in the hole. A front cover 16 is provided to retain the internal gear frame 11. That is, the front cover 16 is secured to the front end face of the yoke 2 and abutted against the outer surface of the flange 13a of the output shaft 13, to prevent the movement of the internal gear frame 11 in the forward direction (to the left-handed side in FIG. 4). The yoke 2 is secured through the front cover 16 to a front bracket 17.
The output shaft 13 is coupled to an over-running clutch, which has a pinion at the front end in such a manner that it is engaged with the ring gear of the engine when the clutch is moved forwardly (not shown).
The conventional planetary speed reduction gear type starter thus constructed operates as follows: Upon energization of the armature winding assembly 8, the rotary shaft 5 is rotated with the aid of the armature. The speed of rotation of the rotary shaft 5 is reduced by the planetary speed reduction gear unit 10. The rotation thus speed-reduced is transmitted to the output shaft 13 to rotate the latter 13. As a result, the over-running clutch is rotated. The over-running clutch is moved forwardly by a shift lever (not shown) with the aid of an electromagnetic switch. Thus, the engine is started through the pinion.
In the above-described conventional planetary speed reduction gear type starter, the diameter of the planetary speed reduction gear unit 10 is determined from the inside diameter of the yoke 2, and the diameter of the motor 1 is inversely proportional to the reduction gear ratio which is determined from the number of teeth of the internal gear 11a. Therefore, in order to increase the reduction gear ratio, it is necessary to increase the size of the planetary speed reduction gear unit to an extent, and accordingly it is necessary to increase the diameter of the yoke 2. However, the increase in diameter of the yoke 2 impedes the miniaturization of the starter.
In order to eliminate the above-described difficulty, a technique of providing the reduction gear outside of the yoke has been proposed. However, the technique still suffers from the following disadvantages: The resultant starter is intricate in construction, and is increased in the number of manufacturing steps. Furthermore, in the case of manufacturing starters different in reduction gear ratio by the technique, it is necessary to form front brackets different in mounting means; that is, the number of kinds of front brackets is increased, thus hindering the standardization of the front bracket.